Membrane osmometers are used for measuring the osmotic pressure exerted by a solution, i.e., a test liquid, on its solvent. One important field of use for membrane osmometers is the measurement of the colloid osmotic pressure of blood, serum, plasma, or other body fluids in comparison to isotonic saline solution.
It is known for the sample chamber, or the cavity formed to receive the test liquid from the sample chamber, to be left open to the atmosphere. It is also known to support the semi-permeable membrane which separates the pressure measuring chamber from the sample chamber or separates the cavities formed by these chambers from one another, by means of a support or sieve plate which is coarsely pervious to liquids, this plate being provided on the pressure measuring chamber side of the membrane to provide rigid fixing for the generally soft, thin membrane. It is also known to use one or more sealing rings, e.g., O-rings, for sealing the pressure measuring chamber and the membrane which closes it off from the sample chamber or from the atmosphere. It is further known to give the support plate a convex curvature directed towards the sample chamber to promote contact or abutment of the membrane on the support plate even if there is no osmotic differential pressure.
For the purpose of holding the sample chamber and pressure measuring chamber together, typically a screw ring which is fitted, as a screw cap, over one of the chambers and engages an external thread on the other chamber, is used.
Semi-permeable membranes are generally water-permeable, to a greater or lesser extent, both at right angles to the surface of the membrane and also parallel thereto. In socalled asymmetrical membranes, the semi-permeable layer (from about 0.1 to 10 .mu.m thick) is only a thin surface coating of the membrane structure; the rest of the cross-section of the membrane (from about 30 to 100 .mu.m thick) is a support structure and is deliberately highly water-permeable in all directions. The semi-permeable layer is generally arranged nearest the sample chamber to facilitate rinsing and cleaning by the test liquid and to prevent, for example, proteins from being trapped in the support fabric of the membrane.
The sealing of the pressure measuring chamber, which is particularly important to the operation of the osmometer, must therefore be effected between the top of the membrane--near its outer edge--and the pressure measuring chamber. Other arrangements for the seal are unsatisfactory or impractical. To ensure maximum stability of volume of the pressure measuring chamber, on which both the duration of measurement and also the accuracy of measurement depend, particularly when the test samples are small, it is important that the membrane is already fitted as tightly as possible against the support plate at the start of measurement. Therefore, it is not enough simply to shape the membrane over the more or less convex support plate by pressing substantially at right angles to the surface of the membrane when the sample chamber and pressure measuring chamber are fitted together; rather, if possible, tension should be produced parallel to the surface of the membrane, towards the edge thereof.